A battery charger includes an attachment portion capable of being connected to a plurality of battery packs; a connection unit for connecting one of the battery packs connected to the attachment portion to a charging path; a state detection unit for detecting a state of each of the battery packs; a remaining capacity detection unit; and a charging control unit. The remaining capacity detection unit detects a remaining capacity based on the state of the battery pack detected by the state detection unit after each of the battery pack is charged for a specified period of time. Further, the charging control unit assigns priorities to the battery packs such that one whose remaining capacity detected by the remaining capacity detection unit is closer to a fully charged state thereof has a higher priority, and controls the connection unit to charge the battery packs according to the assigned priorities.